1. Field of the Invention
This invention relates to a process for electrolytically copper-plating workpieces in a copper sulfate plating bath.
2. Description of the Related Art
Upon forming a pattern on printed circuit boards or wafers, copper sulfate electroplating is applied using an insoluble anode or soluble anode. In a copper sulfate plating bath for the printed circuit boards or wafers, organic additives called a “brightener”, “leveler”, “promoter”, “inhibitor” and/or the like are contained. It is, however, known that in the course of continuous plating, these organic additives may be decomposed or modified to result in a failure in obtaining copper plating films or copper plating fills as desired. This failure is considered to be attributable to decompositions or modifications of the organic additives while oxidation and reduction reactions are repeated at an anode and a cathode in the plating (the thus-decomposed or modified compounds may hereinafter be also called “decomposed/modified organic products”). As art related to copper sulfate electroplating, the following related art can be mentioned, for example.
Japanese Patent Laid-open No. Hei 3-97887 discloses a technique to replenish copper ions to a copper sulfate plating bath, air agitation is performed in a separate tank with unpowered copper metal arranged therein such that the metal copper is dissolved. This patent document, however, makes no mention about a measure for the problem of decomposed/modified organic products.
To solve the above-described problem of organic additives, the following related art has been proposed, for example.
Japanese Patent Laid-open No. 2003-55800 discloses a technique in which dummy plating is performed in a separate tank while using an insoluble anode, such that with oxygen produced from the insoluble anode, decomposed/modified organic products are oxidatively decomposed to lower their content. When plating is continuously performed, however, it takes an unduly long time for the full oxidative decomposition of decomposed/modified organic products. This technology is, therefore, not suited for practical use.
Japanese Patent Laid-open No. 2004-143478 discloses a technique in which air agitation is performed in a separate tank to raise the quantity of dissolved oxygen in a copper sulfate plating bath, such that with the dissolved oxygen, decomposed/modified organic products are oxidatively decomposed. With air agitation alone, however, the oxidative decomposition of decomposed/modified organic products is insufficient. To overcome this problem, it may be contemplated to intensify the air agitation. However, stronger air agitation result in more return of large bubbles to the plating bath. When such large bubbles mix into the plating tank, they adhere to workpieces and cause plating defects such as bare spots. Further, the dissolved oxygen in the plating bath also decomposes organic additives as mentioned above. An excessive increase in the quantity of dissolved oxygen in the plating bath, therefore, leads to greater formation of undesired organic substances. As a consequence, defective copper plating fill and/or defective plating films is induced, and moreover, more organic additives have to be replenished. This technology is, therefore, uneconomical.
Japanese Patent Laid-open No. 2005-187869 discloses to arrange unpowered metal copper in a separate tank and to apply air agitation to the metal copper such that decomposed/modified organic products are decomposed. This technology is, however, uneconomical because a great deal of metal copper has to be arranged to effect the decomposition. In addition, this technology applies air agitation to the metal copper, and therefore, is accompanied by the same problem as the above-described technology of Japanese Patent Laid-open No. 2004-143478.